The present invention relates to a method and apparatus for determining the presence of and correcting and compensating for errors in amplitude encoded signals, and particularly, amplitude encoded angular position signals.
Typically, amplitude encoded angular position signals are generated by angular position encoders which determine the angular position of, for example, a shaft, with respect to a reference angle, through a mathematical computation from two amplitude modulated signals wherein the amplitudes are quadrature related (90.degree. out of phase) sinusoidal signals. Conventionally, an input periodic signal sin wt is provided to an input coil of the position encoder. Two output coils generate respective output signals (A.sub.1 sin .THETA..sub.m) sin wt and (A.sub.2 cos .THETA..sub.m) sin wt in response to rotation of the shaft. A sinusoidal function of the shaft angular position .THETA..sub.m thus amplitude modulates the periodic signal sin wt. In the absence of any errors or any inaccuracies in the angular position encoder and the circuitry coupled thereto, ideally A.sub.1 =A.sub.2, and therefore, one can determine the angular position .THETA..sub.m as the Arctan (sin .THETA..sub.m /cos .THETA..sub.m). If errors are present, for example, due to manufacturing tolerances in the position encoder, one might have two different gains in the two output channels, an offset between the channels or cross coupling between the two channels. This may make A.sub.1 .noteq.A.sub.2 and may provide other added error terms to the channel output signals, for example, offset errors between the two signals from the output coils of the position encoder or second order error signals, e.g., due to cross coupling or gain difference.